Background: Chimeric antigen receptor (CAR)-engineered T cell therapies are being pursued for the treatment of mCRPC. Prostate Stem Cell Antigen (PSCA) is highly expressed on the surface membrane in mCRPC and with limited expression on normal tissues. We are completing a phase I study of PSCA-targeted 4-1BB-co-stimulated CAR T cell therapy in mCRPC. Methods: CAR T cells were manufactured at City of Hope’s cGMP facility. Patients (pts) were required to have disease progression after at least 1 androgen receptor targeted therapy but there was no limit on other prior treatments. The primary objective was to define the dose limiting toxicities (DLT) as well as to describe preliminary bioactivity and activity. CAR T persistence and cytokine levels were evaluated by flow cytometry and Luminex. Circulating tumor cells (CTC) were measured by an unselected assay (Kuhn). Results: 14 pts were treated, median age 69, median baseline PSA 88. 12(86%) had previously been treated with chemotherapy; 9 received two taxanes and 3 docetaxel only. 79% of screened pts had PSCA expression >2+ in >80% of cancer cells. Two of 6 pts encountered DLT (grade 3 cystitis) at 100M + lymphodepletion (LD) chemotherapy. The protocol was amended to reduce the LD dose to 300 mg/m2 cyclophosphamide D1-3. No DLTs occurred in 5 pts treated in the modified LD 100M cohort. Cytokine release syndrome (CRS), best response by RECIST, CAR T cell expansion and persistence are presented by dose level in the table. CAR T expansion was greater with LD than without. PSA declines were seen, as well as radiographic improvement, though RECIST response was limited to stable disease (SD) by concurrent bone metastases. Cytokine peaks were higher in patients with anti-tumor effect noted. CTC decreases in both marrow and peripheral blood were seen, associated with PSA declines. Conclusions: PSCA-CAR T cells have anti-cancer activity in mCRPC with DLT of cystitis. LD was required for greater expansion and activity; lower dose LD improved toxicity without clear negative impact on expansion. Clinical trial information: NCT03873805.